Field
Embodiments of the present disclosure generally relate to molten metal anode solid oxide fuel cells (MMA-SOFCs) and, more specifically relate to a MMA-SOFC system which includes a second MMA-SOFC configured to generate electricity from a metal sulfide byproduct.
Technical Background
As is conventionally known, a fuel cell consists of three major parts; an anode, where electrochemical oxidation takes place, a cathode, where electrochemical reduction takes place and the electrolyte membrane, which is a dense, gas impermeable, ion transport membrane which exhibits purely ionic or mixed ionic-electronic conductivity at a specific temperature range. Cathodes produce oxygen ions which then migrate through the electrolyte membranes to the anode electrode. The oxygen ions oxidize the fuel in the anode and thereby produce electrons, which flow through an external electrical circuit back to the cathode, thereby generating electrical energy.
Referring to FIG. 1, conventional molten metal anode solid oxide fuel cells 10 (MMA-SOFCs), include a molten metal anode 40. The molten metal 41 (also called molten metal bath or molten melt), which is oxidized in the molten metal anode 40, is kept in liquid phase by maintaining the operating temperature above the melting point of the metals and metal oxides 42 therein. The molten metal anode 40 is in contact with the solid electrolyte 30 and may also be exposed to fuel (gas, liquid or solid), for example, a sulfur containing fuel. In the cathode 20, which performs O2 (g) reduction of the cathode metal in the presence of air to yield oxygen ions, is placed on the opposing side of the solid electrolyte 30. For current collection at the anode 40, a metal wire 72, or any other electron conducting material that is solid and inert at the operating conditions, may be immersed in the anode melt 41 to facilitate collection of the electrons which travel back to the cathode 20 via electrical circuit 70.
Referring again to FIG. 1, a metal/metal oxide cycle is used such that metal is electrochemically oxidized to metal oxide in the anode 40 and is then reduced by the fuel. This reduction by the fuel regenerates the metal from the metal oxide. When sulfur compounds exist in the fuel, metal sulfides 46 form and may detrimentally inhibit the metal oxide 42 reduction in the molten metal 41 by the fuel. Sulfur contaminations in the fuel can also degrade the performance of the MMA-SOFC and even poison the cell, due to the formation of metal sulfides. Metal sulfides, which form at the fuel/anode interface, are lighter than both the molten metal 41 and the metal oxides 42 and thus float to the top of the molten metal 41 bath. This inhibits the reduction of the metal oxide 42 species by the fuel, as well as the anode recycling and fuel cell operation.
Accordingly, ongoing needs exist for MMA-SOFC systems which provide improved handling and utilization of sulfide byproducts.